A thixo-casting process is drawing researcher's attention these days since it involves a fewer molding defects and segregations, produces uniform metallographic structures and features longer mold lives but shorter molding cycles than the existing casting techniques. The billets used in this molding method (A) are characterized by spheroidized structures obtained by either performing mechanical or electromagnetic agitation in temperature ranges that produce semisolid metals or by taking advantage of recrystallization of worked metals.
On the other hand, raw materials cast by the existing methods may also be molded in a semisolid state. There are three examples of this approach; the first two concern magnesium alloys that will easily produce an equiaxed microstructure and Zr is added to induce the formation of finer crystals [method (B)] or a carbonaceous refiner is added for the same purpose [method (C)]; the third approach concerns aluminum alloys and a master alloy comprising an Al-5% Ti-1% B system is added as a refiner in amounts ranging from 2-10 times the conventional amount [method (D)]. The raw materials prepared by these methods are heated to temperature ranges that produce semisolid metals and the resulting primary crystals are spheroidized before molding.
It is also known that alloys within a solubility limit are heated fairly rapidly up to a temperature near the solidus line and, thereafter, in order to ensure a uniform temperature distribution through the raw material while avoiding local melting, the alloy is slowly heated to an appropriate temperature beyond the solidus line so that the material becomes sufficiently soft to be molded [method (E)]. A method is also known, in which molten aluminum at about 700.degree. C. is cast to flow down an inclined cooling plate to form partially molten aluminum, which is collected in a vessel [method (F)].
These methods in which billets are molded after they are heated to temperatures that produce semisolid metals are in sharp contrast with a rheo-casting process (G), in which molten metals containing spherical primary crystals are produced continuously and molded as such without being solidified to billets. It is also known to form a rheo-casting slurry by a method in which a metal which is at least partially solid, partially liquid and which is obtained by bringing a molten metal into contact with a chiller and inclined chiller is held in a temperature range that produces a semisolid metal [method (H)].
Further, a casting apparatus (I) is known which produces a partially solidified billet by cooling a metal in a billet case either from the outside of a vessel or with ultrasonic vibrations being applied directly to the interior of the vessel and the billet is taken out of the case and shaped either as such or after reheating with r-f induction heater.
However, the above-described conventional methods have their own problems. Method (A) is cumbersome and the production cost is high irrespective of whether the agitation or recrystallization technique is utilized. When applied to magnesium alloys, method (B) is economically disadvantageous since Zr is an expensive element and speaking of method (C), in order to ensure that carbonaceous refiners will exhibit their function to the fullest extent, the addition of Be as an oxidation control element has to be reduced to a level as low as about 7 ppm but then the alloy is prone to burn by oxidation during the heat treatment just prior to molding and this is inconvenient in operations.
In the case of aluminum alloys, about 500 .mu.m is the crystal grain size that can be achieved by the mere addition of refiners and it is not easy to obtain crystal grains finer than 200 .mu.m. To solve this problem, increased amounts of refiners are added in method (D) but this is industrially difficult to implement because the added refiners are prone to settle on the bottom of the furnace; furthermore, the method is costly. Method (E) is a thixo-casting process which is characterized by heating the raw material slowly after the temperature has exceeded the solidus line such that the raw material is uniformly heated and spheroidized. In fact, however, an ordinary dendritic microstructure will not transform to a thixotropic structure (in which the primary dendrites have been spheroidized) upon heating. According to method (F), partially molten aluminum having spherical particles in the microstructure can be obtained conveniently but no conditions are available that provide for direct shaping. What is more, thixo-casting methods (A)-(F) have a common problem in that they are more costly than the existing casting methods because in order to perform molding in the semisolid state, the liquid phase must first be solidified to prepare a billet, which is heated again to a temperature range that produces a semisolid metal. In addition, the billets as the starting material are difficult to recycle and the fraction liquid cannot be increased to a very high level because of handling considerations.
In contrast, method (G) which continuously generates and supplies a molten metal containing spherical primary crystals is more advantageous than the thixo-casting approach from the viewpoint of cost and energy but, on the other hand, the machine to be installed for producing a metal material consisting of a spherical structure and a liquid phase requires cumbersome procedures to assure effective operative association with the casting machine to yield the final product. Specifically, if the casting machine fails, difficulty arises in the processing of the semisolid metal.
Method (H) which holds the chilled metal for a specified time in a temperature range that produces a semisolid metal has the following problem. Unlike the thixo-casting approach which is characterized by solidification into billets, reheating and subsequent shaping, the method (H) involves direct shaping of the semisolid metal obtained by holding in the specified temperature range for a specified time and in order to realize industrial continuous operations, it is necessary that an alloy having a good enough temperature distribution to establish a specified fraction liquid suitable for shaping should be formed within a short time. However, the desired rheo-casting semisolid metal which has spherical primary crystals, a fraction liquid and a temperature distribution that are suitable for shaping cannot be obtained by merely holding the cooled metal in the specified temperature range for a specified period. Too rapid cooling will deteriorate the temperature distribution. In addition, if the cooling means is contacted by the melt, a solidified metal will remain either on the cooling means or within the holding vessel, making it impossible to perform continuous operation.
In method (I), a case for cooling the metal in a vessel is employed but the top and the bottom portions of the metal in the vessel will cool faster than the center and it is difficult to produce a partially solidified billet having a uniform temperature distribution and immediate shaping will yield a product of nonuniform structure. What is more, considering the need to satisfy the requirement that the partially solidified billets as taken out of the billet case have such a temperature that the initial state of the billet is maintained, it is difficult for the fraction liquid of the partially solidified billet to exceed 50% and the maximum that can be attained practically is no more than about 40%, which makes it necessary to give special considerations in determining injection and other conditions for shaping by diecasting. If the fraction liquid of the billet has dropped below 40%, it could be reheated with a r-f induction heater but it is still difficult to attain a fraction liquid in excess of 50% and special considerations must be made in injection and other shaping conditions. In addition, eliminating any significant temperature uneveness that has occurred within the partially solidified billet is a time-consuming practice and it is required, although for only a short time, that the r-f induction heater produces a high power comparable to that required in thixo-casting. In addition it is necessary to install multiple units of the r-f induction heater in order to achieve continuous operation in short cycles.
Another problem with the industrial practice of shaping semisolid metals in a continuous manner is that if a trouble occurs in the casting machine, the semisolid metal may occasionally be held in a specified temperature range for a period longer than the prescribed time. Unless a certain problem occurs in the metallographic structure, it is desired that the semisolid metal be maintained at a specified temperature; in practice, however, particularly in the thixo-casting process where the semisolid metal is held with its temperature elevated from room temperature, the metallographic structure becomes coarse and the billets are considerably deformed (progressively increase in diameter toward the bottom). In addition, unless their temperatures are individually controlled, such billets are usually discarded and cannot be used as thixo-billets.
The present invention has been accomplished under these circumstances of the prior art and its principal object is to provide an apparatus that does not require to use billets or any cumbersome procedures but which ensures that semisolid metals (including those which have higher values of fraction liquid than what are obtained by the conventional thixo-casting process) which are suitable for subsequent shaping on account of both a uniform structure containing spheroidized primary crystals and uniform temperature distribution can be produced in a convenient, easy cost-effective way. In addition, if the need arises to control the semisolid metal by holding it at a specified temperature during prolonged machine trouble or in the case where a semisolid metal having a specified fraction liquid is rapidly produced to permit high shot-cycle operations and where it is adjusted to fall within a specified temperature range prior to molding, the apparatus is capable of producing a semisolid metal suitable for semisolid shaping by holding the metal's temperature uniformly at a constant level with such great rapidity that the power requirement of the r-f induction heater is no more than 50% of what is commonly spent in shaping by the thixo-casting process.